Stiles’ reagent

A carbonic acid derivative which, surprisingly, also proves to be suitable for the acylation of ketone enolates, is Stiles reagent, i.e., (methoxymagnesium) monomethyl carbonate. In Section 8.2, you saw how this reagent can be obtained. Ketone enolates are carboxylatedby Stiles reagent to furnish a /3-keto carboxylic acid, as shown by the reaction equation below. As this keto acid is initially obtained as the (methoxymagnesium) carboxylate, such an acylation can easily proceed without the extra equivalents of enolate or base mentioned above. 

Fig. 13.63. -Carboxylation of a lactone enolate with (methoxymagnesium) carbonic acid monomethyl ester ("Stiles reagent"). In the C,C bondforming step (C -> E) this anion acts as an electrophile, since it is steered into close vicinity to the enolate carbon by a Mg2 cation.

Acrylic monomers can be derived from biomass (53). An early synthesis of a-methylene-y-valerolactone involves two steps (54,55). The first step is the carboxylation of y-butyrolactone with methyl methoxymagnesium carbonate Stiles reagent) to produce the acid. Next, the acid is briefly treated with a mixture of aqueous formaldehyde and diethylamine, followed by a separate treatment of the crude product with sodium acetate in acetic acid. The first step requires 6-7 h and affords almost quantitative yields, whereas the second step can be accomplished in less than 30 min but with yields of only 50%. 

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